Electrical resistor and method of producing the same



R. C. RAGAN May 24, 1966 ELECTRICAL RESISTOR AND METHOD OF PRODUCING THESAME Filed May 6, 1964 PI G. 2

Fl G. 1

FIG. 4

FIG. 3

I IIIIIII I Tern perafure Temperature FIG. 6

FIG. 5

INVENTOR.

RANDALL C. RAGAN BY M54 United States Patent i 3,252,831 ELECTRICALRESISTOR AND METHOD OF PRODUCING THE SAME Randall C. Ragan, Tarzana,Califi, asslgnor to Electra Manufacturing Company, Independence, Kans.,a corporation of Missouri Filed May 6, 1964, Ser. No. 365,399 17 Claims.(Cl. 117217) This application is a continuation-in-part of my copendingapplication Serial No. 779,605, Precision Electrical Circuit Elements,filed December 11, 1958, which in turn is a continuation-in-part ofapplication Serial No. 618,728, Precision Electrical Circuit Elements,filed October 29, 1956, now abandoned.

The present invention relates generally to electrical resistors and,more particularly, to an improved film type resistor and a method ofproducing the same.

Heretofore, a great variety of different film-type resistors have beenproposed. In general, these resistors are formed by depositing thinmetal films on electrically insulating base materials, such as bycathode sputtering, vacuum evaporation, or electrodeposition processes.More recently, it has been proposed to form such resistors of variousmetal-glass films, withthe glass serving as a binder for theelectrically conductive metal particles. However, the metal-glass filmsdeveloped thus far have exhibited a number of serious shortcomings. Forexample, the rnetal-glass films previously proposed are gen erally ofnon-uniform composition through the film thickness so that abrading ofthe film surface, either during manufacture or during wear, changes thecharacteristics of the film. Moreover, it has been found that heating ofthe metal-glass film to the temperature required to fuse the glassincreases the resistance of the film considerably, so that the film musteither be made relatively thick or made with a relatively highpercentage of metal. Also, perhaps more important than any of the othershortcomings is the fact that .difliculties have been encountered inproducing films with properties which are predictable and stable.

It is a primary object of the present invention to provide an improvedmetal-glass film resistor which is capable of being reproduced withaccurately predictable electrical resistance, temperature coefiicient ofthermal expansion, and other characteristics. A related object of theinvention is to provide a resistor of the foregoing type which is stableelectrically, chemically and physically. Thus, it is an object toprovide such a resistor which is characterized by high load lifestability and good voltage coeflicient of-electrical resistivity, i.e.,the resistance remains constant regardless of the magnitude of thevoltage applied, assuming no self heating.

It is another object of the present invention to provide an improvedmetal-glass film resistor which is of substantially uniform compositionthrough its thickness. In this connection, it is a particular object ofthe invention to provide a metal-glass film resistor which can beabraded without changing its characteristics,' other than the change inresistance due to the decrease in cross sectional area.

. A- further object of the invention is to provide an improved method ofproducing a metal-glass film resistor of the foregoing type whereby theglass component of the film may be fused without increasing theresistance of the film. A related object is to provide such a methodwhich permits the attainment of any given resistance with a thinner filmand/ or a lower percentage of metal than the methods of the prior art.

It is still another object of the invention to provide an improvedmetal-glass film resistor which can be manu 3,252,831 Patented May 24,1966 factured in a wide range of electrical resistance values, and at alow cost. Yet another object of the invention is to provide such aresistor which is characterized by very low noise, and which is ideallysuited for use as the resistor element in a potentiometer.

Other objects and advantages of the invention Will become apparentuponreading the following description and appended claims and upon referenceto the drawings, in which:

FIGURE 1 is a plan view of a film-type electrical resistor embodying thepresent invention;

FIG. 2 is a section taken along line 22 in FIG. 1;

FIG. 3 is a temperature-resistivity curve showing the variations inelectrical resistivity with changes in temperature during themanufacture of a metal-glass film resistor according to the methods ofthe prior art;

FIG. 4 is a temperature-resistivity curve showing the variations inelectrical resistivity with changes in temperature during themanufacture of a metal-glass film resistor according to the method ofthis invention;

FIG. 5 is a plan view of a linear potentiometer made in accordance withthis invention; and

FIG. 6 is a plan view of a nonlinear potentiometer made in accordancewith the invention.

While the invention will be decribed in connection with a preferredembodiment, it is to be understood that the invention is not to belimited to the disclosed embodiment but, on the contrary, it is intendedto cover the various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claims.

Turning now to the drawings, in FIGURE 1 there is shown a film-typeelectrical resistor including a substrate 10 formed of an electricallyinsulating material which is resistant to high temperatures, and a thinfilm 11 of an electrical resistance material comprising at least onenoble metal and a low melting glass bonded to the surface of thesubstrate 10. As used herein, the term noble metal includes gold,silver, palladium, platinum, rhodium, and iridium. The substrate 10 maybe made of any suitable thermally stable substrate material such as, forexample, alumina, steatite, fosterite, glass, porcelain, mica and otherceramic materials having the necessary physical, chemical, andelectrical properties forthe particular use intended. The substratemust, of course, be capable of withstanding the action of any chemicalsused in the deposition of the film 11, as well as any changes intemperature encountered both'during the manufacturing process and duringuse of the final product. For the purpose of connecting the resistorelement into the desired circuit, a pair of highly conductive metalterminals 13 are deposited on the substrate 10 in electrical'contactwith opposite ends of the film 11'. The terminals 13 may be made of anysuitable metal, such as silver for example,

In forming the metal-glass film 11.1, a finely divided metal-glassmixture of the desired composition is deposited on the surface of thesubstrate and then fired to a temperature sufficiently high to fuse theglass component of the mixture, but below the melting point 'of themetal component. The metal-glass system is then held at that temperaturefor a period sufiicient to form a continuous glassy film which isbonded. firmly to the substrate. As the metal-glass system is fired, itselectrical resistance has been found to follow a general pattern. Thus,atypical "metal-glass system hasi'a temperature-resistance curve asillustrated in FIG. 3. The electrical resistance of the metal-glasssystem as initially deposited on the insulating substrate, i.e., at.temperature T is essentially infinite, especially where a relatively'high percentage of glass is employed. As the temperature of themetal-glass system is increased, its electrical resistance decreasesrather rapidly until the curve becomes-asymptotic to the temperatureaxis, as at temperature T This represents the minimum resistance valueof the metal-glass system, and further increases in temperature causethe resistance to increase at a rather rapid rate.

In order .to form a continuous glassy film from the metal-glass system,it must be heated at least to the fusion point of the glass component ofthe system, which is temperature T in FIG, 3. However, as can be seenfrom the curve, if the temperature is increased from T to T at the samerate as from T to T the resistance increases to a level considerablyabove the minimum value attained at T The resistance dips somewhat atthe glass fusion point, but it is still well above the resistance valueat T Consequently, the thickness of the final film must be considerablygreater than the thickness which would be required with the resistancevalue attained at T Since one of the main objects of film-type resistorsis to minimize the size of the resistor, it would naturally be desirableto produce a film in which the glass component has been fused attemperature T but which still hasthe relatively low resistance valueattained at T Moreover, it has been found that the results of such aheating treatment are unpredictable in that the characteristics of thefinal resistor are largely indeterminate.

In accordance with the present invention, it has been unexpectedlydiscovered that the objectionable increase in resistance of themetal-glass film. between T and T can be substantially avoided byinitially increasing the temperature of the system at a relatively slowrate from T to T increasing the temperature from T to T at a relativelyrapid rate, and then maintaining the system at T for a period justsufficient to complete the fusion of the glass component withoutdegrading the system. Thus, it has been surprisingly found that bycarrying out the firing process in two stages, with a relatively rapidheating rate being used in the second stage, the glass component of thesystem can be completely fused without increasing the electricalresistance of the system. Indeed, if the heating rate in the secondstage is properly controlled, the resistance of the film may actually bedecreased below the resistance value attained at T Furthermore, thecharacteristics ofresistors produced by this two-stage process areaccurately predictable so that any given resistor can. be faithfullyreproduced time. after time.

Referring to FIG. 4, which illustrates a temperatureresistance curve fora typical metal-glass film produced by the method of this invention, itcanv be seenv that the first stage of the firing process, i.e., from Tto. T is similar to the corresponding portion of the curve of FIG.

3. From T to T however, the tempera-tureis increased at such a r-apidrate that the resistance remains the same as, or even drops: below, theresistance. value at T It will be understood that theexact temperaturesrepresented by T and T and. also the, exact heating rates employed inthe two stages, will vary with the composition of the metal-glass film.Forexample, in the case of a metalglass system comprising a. glasscomponent of lead-borosilicate glass and, a ternary metal component ofsilver,

gold, and platinum resinates, a suitable, procedure for the first stageof the process is to. place. a. substrate coated with the metal-glassmixture in an ovenv at 250 F. and then increase the temperature: toabout 720 'F. in about fifteen minnes. This decomposes the. metalresinates, depositing the three noble metals on. the substrate anddriving offthe volatile decomposition products, and produces ametal-glass film. having the minimum electrical resistance for thisparticular system. 1

For the second stage of the process, the resistor element: is removedfrom the first oven and transferredto a second oven which has been.preheated to ab;out:1020 R, which is slightly above the, fusing point-0fthe lead-borosilicate glass but below, thev melting, points of the threenoble metals. Since the mass of the resistor element is small, thetemperature ofthe metal-glass film is, increased to the temperature ofthe second furnace in amatter of aasasai seconds, thereby fusing theglass component with practically no increase in the electricalresistance ofthe system. The resist-or element is allowed to remainin'the second furnace for about 4 to 6 minutes, which is sufficient toinsure complete fusion of the glass 'Without changing the resistance orother electrical properties of the film. Below four minutes, the' filmdoes not mature, i.e., there may be inadequate fusion. Above sixminutes, there is an increase in the noise level of the resistorelement, probably because the conductive metal phase of the film beginsto become discontinuous.

In order to achieve the desired glassy coating, the metal-glass systemmust be fired at least to its fusing point. However, overfiring maycause bubbling or blistering of the glass component of the system and,therefore, is to be avoided. The fusion temperature must also be belowthe melting point of the metal component of the system. The firingtemperature employed in any given case depends on the particularcomposition used, and it will be understood that different firingprocedures may be devised according to the teachings of this invention.

In accordance with one aspect of this invention, the metal component ofthe metal-glass system comprises a ternary systemof silver, platinum,and gold, either alone or in combination with one or more metal oxidessuch as the oxides of iridium, ruthenium, palladium, rhodium, nickel,cobalt, iron, manganese, chromium, vanadium,

and titanium. The preferred ternary systems are those containing about25 to 65% by weight silver, 20 to-45% by weight platinum,-and 5 to 35%by weight gold. The silver-platinum-gold ternary system has been foundto produce a highly conductive film with accurately predictable andreproducible characteristics. By contrast, it has. been found thatsilver and gold, when used alone, are practically non-conductors, andplatinum when used alone is a poor conductor. Similarly, most binarysystems are also unsuitable for use in this invention; thus, asilver-gold system is not sutficiently conductive and a platinum-silversystem is diffi-cult to reproduce with uniform properties. One binarysystem which may be used is a platinum-gold system, with the majorportion gold, but this is still inferior to the ternary system.

In addition to the high degree of predictability, the metal-glassresistors made from the preferred silverplatinum-gold system accordingto this invention have been found to be extremely stable electrically,chemically, and physically. Thus, these films have high loadlifestability and good voltage coefficient of electrical resistivity, i.e.,the resistance remains constant regardless of the magnitude of thevoltage applied (assuming no self heating). Moreover, the temperaturecoeficient of electrical resistivity is always positive and predictable,being 200 parts per million positive, plus or minus only about 20 partsper million. These preferred films also, exhibit .low noise (too low tomeasure in the low resistance values), are. physically strong, andchemically inert.

Although the explanation of the vastly superior performance of thesilver-platinum-gold system is not completely understood, it isbelieved. that one or two metals existing alone in the metal-glassmatrix tend -to' coalesce or cohere so that instead of having a largenumber of tiny metal particles evenly spaced, there are produced largeirregularly shaped' masses of conductive material surrounded by theinsulating glass. In the preferred ternary system, however, each metalseems to prevent the others from coalescing so that the metalparticlestend to remain separated andeve nly distributed, thereby forming acontinuous phase metal-glass alloy. Thus, the resulting metal-glassfilm, isuniformly'conductive throughout and may be faithfully duplicatedin mass production with exactly the desired characteristics. Moreover,it

has been surprisingly found that silver, platinum, and gold are the onlynoble metals which are nonoxidizable in the method of this invention.

- lead oxide (PbO).

borate glasses have been found to produce non-uniform As mentionedabove, the preferred silver-platinurn-gold ternary system may bemodified with certain additives.

Especially valuable additives are the oxides of nickel, cobalt, andcopper. Although these oxides are highly soluble in the glass matrix,they have unexpectedly been found to reduce the temperature coeflicientof electrical resistivity of the metal-glass film, to increase theelectrical,

resistance of the film, and to improve its abrasion resistance.

for the glass component is a low melting lead-boro-silicate glass, suchas a glass composed of 75% PhD, B 0 and 10% SiO This glass melts at 900E, which is below the melting points of the noble metals, and remainsstable over extended periods of operation at elevated temperatures. Ingeneral, the preferred lead-bor-o-silicate glasses are those containingat least about 75% by weight Both the lead-silicate and the leadof theresistor either in the process of manufacture or in use, the coefiicientof thermal expansion of the glass component should be adjusted tocorrespond with the coetficient of thermal expansion of the substrate towhich the film is bonded. For example, in the case of the preferredlead-boro-silicate glass, the coefficient of thermal expansion may beadjusted by the addition of zirconium oxide (ZrO). the coefficient ofthermal expansion, but also produces a mechanical matrix for the glassso that the glass becomes less fluid and thus less likely to flow. Theglass may also be modified to improve its acid and alkali resistance,such as by the addition of zirconium oxide or titanium oxide (TiO tolead-boro-silicate glass. The addition of these modifying agents isusually accomplished by the addition of active fiuxes, such as alkalimetal oxides or fluorides,

to the boric glass composition.

The electrical resistivity of the metal-glass film of this inventiondepends not only on the firing procedure, but also on the particularproportions of metal and glass employed, the dimensions of the film, andthe composition of the particular metal and glass components employed.It is generally preferred to use less than about 80% by weight glass,suitably about 50% by weight, but higher percentages may be used forcertain applications. The exact thickness of the metal-glass film will,of course, vary with different resistors, but the thickness willgenerally be less than five mils, preferably within the range of 0.0001to 0.005 inch.

Although the metal-glass films are resistant to moisture, neverthelessit is often desirable to provide a protective coating over the resistorfilm. In this connection, the metal-glass films of this inventionbecause of their inertness are capable of tolerating a wider range ofprotective coatings than other types of resistors. Typical coatingmaterials include glass of the same composition as the matrix andcertain lacquers. The coating material should not include any catalysts,such as epoxy resins and silicones with a drier or polymerizing agent.

In order to deposit a uniform and accurately reproducible metal-glassfilm, it is preferred to intimately mix the metal and glass componentsin a liquid vehicle which is then applied to the substrate. Oneparticularly preferred method is to mix the finely ground glasscomponent with the appropriate metals in the forms of soluble metalresinates which are thermally decomposable. The metal resinates'aredissolved in a suitable solvent and intimately The zirconium oxide notonly corrects mixed with the finely ground glass to form a uniformmixture which can be easily applied to the desired substrate by asuitable stencilling technique, such as silk screening for example, orby the application of ordinary printing, engraving, and lithographingtechniques and the like. The metal resinates and the glass flux shouldbethoroughly mixed, such as in a conventional three-roll paint mill forexample. In order to provide proper screenprinting consistency and toimprove the ignition properties of the resinates, a plasticizer such ashydroabietyl alcohol or the like may be added to the mixture.

After. the mixture has been deposited on the substrate, the coatedsubstrate is then placed in an oven and fired to the decompositiontemperature of the metal resinates. In this particular method, thefiring procedure should be carried out in an oxygen-containingatmosphere so that the carbonaceous decomposition products are oxidizedto form carbon oxides which may pass off in gaseous form rather thanbeing left on the resistor element. This also eliminates any possibilityof the carbon reducing a portion of the glass component to a metallicphase, such as lead oxide to the metal lead for example.

It will be understood that the thermal decomposition of the metalresinates represents only the first stage of the two-stage method ofthis invention, andthe resist-or must still be heated to the fusingtemperature of the glass component of the film at a relatively highheating rate as de scribed above. In order to determine the actualtermination temperature of the first stage of the process, it has beenfound convenient to monitor a resistor element consisting of a mixtureof resinates of the same noble metals and in the same proportions as inthe resistors under treatment, but without the glass component. Thisglass-free mixture provides the lowest possible resistance for a givensystem and thus gives a more sensitive indication of changes in themonitoredresistance value. The resistance starts out at a high level andthen drops rapidly as the resinates are decomposed and the organicmaterial driven off. When the monitored resistance levels'otf at thebottom of the curve, the first stage of the heat treatment is terminatedand the resistor is ready for the second stage treatment.

It will be appreciated that other thermally decomposable organo-metalliccompounds may be used as sources of the noble metals. such compounddecompose completely at a temperature below the fusion point of theparticular glass employed, and that it produce decomposition productswhich are easily volatilized or otherwise disposed of without leavingany deleterious residue. Also, to facilitate liquid application, thecompound should be readily soluble in a solvent which can be evaporatedto leave a stable mechanical configuration. Typical compounds which meetthese requirements include not only the resinates, but also octoates,palmatates, oleates, and many other salts of organic acids. It will beunderstood that the commercial forms of the resinates and othercompounds normally include various fluxes, such as a small amount ofbismuth, to render them more adherent. However, the bismuth or otherflux forms no part of the present reaction and is promptly absorbed bythe glass component.

For the purpose of avoiding unpredictable variations in the resistivity,noise level, and other characteristics of the resistor element, themetal-glass system should be fired in a purified atmosphere and undercontrolled humidity conditions. Thus, when the films are fired inan'oven, for example, the air which is fed to the oven is preferably fedthrough a bed of charcoal to remove various organic gaseous impurities.Moreover, the relative humidity is preferably controlled to stay withinthe range of about 40 to 60% at 75 F., or the preferred moisture contentis between about 5 and 6 grains per cubic foot, regardless oftemperature. I 1 In aseries of examples of the present invention, anumber of different metal-glass films were deposited from It isimportant that any 7 various mixtures of lead-boro-silicate glass (75%PbO, 15% B SiO and silver platinum, and gold resinates. Each mixturecontained about 50% by weight glass and the followingvpercentages byweight of the various metal resinates:

In each example, the mixture was deposited on an alumina substrate,placed in a first oven at 250 F., and heated to 720 F. in about minutes.The resistor was then removed from the first oven and placed in a secondoven which had been preheated to 1020 F. After about 4 to 6 minutes, theresistor was removed from the second oven, allowed to cool to roomtemperature, and tested for electrical resistance temperaturecoefficient of electrical resistivity, and other properties.

The compositions of Examples 1 and 2 produced films which werepractically non-conductive and therefore useless as resistors. Examples3 through 9 produced films with the desired combination of propertiesand were distinguished from each other only by small differences inresistance and temperature coeflicient of electrical resistivity. Allthese films had low noise levels and were not subject to change withvariations in the firing conditions. Moreover, the temperaturecoeflicient of electrical resistivity of all these films was stable. Thefilm of Example 10 exhibited relatively large changes in both resistanceand temperature'coefficient of electrical resistivity, while Example 11was stable and reproducible but too high in temperature coefiicient formost practical applications. Examples 12 and 13 produced films withextremely high resistance values and high and variable temperatureooeificients of electrical resistivity, and were not stable underelectrical load. The last two Examples, 14 and 15, were lower inresistance than 12 and 13 and were relatively stable with acomparatively good combination of properties.

The metal-glass films of this invention have smooth glassy surfaceswhich are especially useful in potentiometer. Thus, in FIG. 5, a linearpotentiometer is formed by depositing a circular metal-glass film ofuniform width and cross section on an insulating substrate 21. For thepurpose of connecting the film 20 into the desired circuit, a pair ofhighly conductive metal. tab-s 22 are deposited on the substrate 21 atthe ends of the film 20. The usual potentiometer slide (not shown) maybe supported rotatably to contact the resistance film 20 at any desiredpoint along ts length. To improve the wear and noise qualities of thepotentiometer, the slide preferably has its contact surface made of aconductive metalglass film.

The potentiometer of FIG. 6 is similar to that of FIG. 5- except thatthe metal-glass resistance film 20a gradually increases in width aroundits circumference so as to form a nonlinear potentiometer. The substrate21, the terminals 22, and the slide arrangement may be the same asdescribed about for FIG. 5'.

; In accordance with one aspect of this invention, a metal-glass filmwhich is especially useful in potentiometer application is produced bycoating the substrate with the glass to be used in the metal-glass film,fusing the glass coating to form a smooth continuous glass compositefilm which is ideal 8 film bonded to the substrate, and then-depositingth metal-glass film in the manner described above. During firing of themetal-glass film, it sinks into the previously deposited glass coatingso as to form an extremely smooth for use as a miniaturizedpotentiometer.

While various specific forms of the present invention have beenillustrated and describe-d herein in some detail, it will be apparentthat the same are susceptible of numerous modifications within thespirit and scope of the invention. For example, instead of proceedingdirectly from the first stage of the firing process to the second stage,the metal-glass film may be allowed to cool to room temperature or someother intermediate temperature between the two stages without anyappreciable effect on the final result. Also, more than one layer of themetalglass film may be deposited on a single substrate, and in anydesired configuration;

I claim as my invention:

1. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which isresistant to high temperatures, depositing a mixture of finely dividedglass and thermally decomposable organic compounds of the metals silver,platinum, and gold as a thin coating on the surface of said substrate,said glass having a fusing point below the melting points of saidmetals, increasim the temperature of said coating at a relatively slowrate until the electrical resistance of the metal-glass system isreduced to substantially its minimum value, increasing the temperatureof said coating at least to the fusing point of said glass, but belowthe melting point of said metals, at a relatively rapid rate sufficientto avoid any substantial increase in the electrical resistance of themetal-glass system, maintaining the coating at the fusing temperaturefor a period sufficient to completely fuse the glass withoutsubstantially increasing the electrical resistance of the metal-glasssystem, and cooling the resulting glassy coating to solidify the glassand form a continuous impervious metal-glass film bonded to the surfaceof the substrate.

2. The method of claim 1 Whereinthe glass component of said metal-glassfilm has a coefiicient of thermal expansion compatible with. thecoefficient of thermal expansion of said electrically insulatingsubstrate.

3. The method of claim 1 wherein the glass component of said metal-glassfilm is a lead-boro-silicate glass.

4. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which has a.smooth surface and is resistant to high temperatures, depositing a.mixture of finely divided glass and a thermally decomposable organiccompound of a noble metal in a liquid vehicle as a thin coating on thesmooth surface of the substrate, said glass having a fusing point belowthe melting point of the noble metal, heating the coating at arelatively slow rate in an oxidizing atmosphere so as to decompose saidorganic com- I pound and reduce the electrical resistance of themetalglass system to substantially its minimum vlaue, heating themetal-glass system at least to the fusing point of the glass component,but below the melting point of the noble metal, at a relatively rapidrate sutficient to avoid any substantial increase in the electricalresistance of the metal-glass system, maintaining the coating at thefusing temperature fora period suflicient to completely fuse the glasswithout substantially increasing the .electrical resistance of themetal-glass system, and cooling the resulting iglassy coating tosolidify the glass and form a continuous impervious metai-glass filmbonded to the surface of the substrate.

5.' The method of claim 4 wherein said heating steps are carried out ina purified atmosphere having a relative humidity of about 40 to at F. v

-6. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which has asmooth surface and is resistant to high temperatures, depositing amixture of finely divided glass and a ternary noble metal resinatesystem consisting essentially of silver resinate, platinum substantialincrease in the electrical resistance of the metal-glass system,maintaining the coating at the fusing temperature for a periodsufficient to completely fuse the glass without substantially increasingthe electrical resistance of the metal-glass system, and cooling theresulting glassy coating to solidify the glass and form a continuousimpervious metal-glass film bonded to the surface of the substrate.

7. A method of producing a film-type electrical resistorcomprising thesteps of providing an electrically insulating substrate which has asmooth surface that is resistant to high temperatures, depositing amixture of about equal parts by weight of a finely dividedleadboro-silicate glass and a ternary noble metal resinate systemconsisting essentially of the resinates of silver, platinum, and gold asa thin coating on the surface of said substrate, heating said coating toabout 720 F. in about 15 minutes so as to decompose the metal resinatesand reduce the electrical resistance of the metal-glass system tosubstantially its minimum value, the heating being carried out in anoxidizing atmosphere so as to oxidize the volatile decompositionproducts from the resinates, heating the resulting metal-glass coatingto about 1020 F. substantially instantaneously so asto fuse thelead-boro-silicate glass without increasing the electrical resistance ofthe metal-glass system, maintaining the metal-glass coating at about1020 F. for about 4 to 6 minutes to completely fuse the glass Withoutincreasing the electrical resistance of the metalglass system, andcooling the resulting glass coating to solidify the glass and form acontinuous impervious metalglass film bonded to the surface of thesubstrate.

8. Afilm-type electrical resistor comprising the combination of anelectrically insulating substrate which is resistant to high temperatires, and a metal-glass film deposited on the surface of aid substrateand having a glass component with a fusing ;oint below the melting pointof the metal component, sa d glass component having been fused to form acontim ous impervious metal-glass film bonded to the surface of thesubstrate, and a metal component consisting essentially of silver,platinum, and gold.

9. A film-type electrical resistor comprising the combination of anelectrically insulating substrate which is resistant to hightemperatures, and a metal-glass film deposited on the surface of saidsubstrate and having a glass component with a fusing point below themelting point of the metal component, said glass component having beenfused to form a continuous impervious metal- -glass film bonded to thesurface of the substrate, and a metal component consisting essentiallyof about 25 to 65% by Weight silver, about 20 to 45% by weight platinum,and about to 35% by Weight gold.

10. The film-type electrical resistor of claim 9 wherein the glasscomponent of said metal-glass film is lead-borosilicate glass.

11. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which isresistant to high temperatures, depositing a thin glass coating on thesurface of said substrate and fusing the glass coating so as to form asmooth continuous glass film bonded to the substrate, solidifying theglass coating, depositing a mixture of finely divided glass andthermally decomposable organic compounds of the metals silver, platinum,and gold as a thin coating on the surface of the solidified glasscoating, said finely divided glass having substantially the samecomposition as the initial glass coating and also having a fusing pointbelow the melting point of said metals, increasing the temperature ofsaid metal-glass mixture to at least the fusing point of theglass, butbelow the melting point of said metals, so that the metal-glass mixturepenetrates into the initial glass coating forming a smooth surfacedcomposite metal-glass film, and cooling said composite film to solidifythe glass and form a continuous impervious metal-glass film.

12. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which isresistant to high temperatures, depositing an initial glass coating onthe surface of the substrate and fusing said glass to form a continuousglass film, solidifying the fused glass, depositing a mixture of finelydivided glass and thermally decomposable organic compounds of the metalssilver, platinum, and gold as a thin coating of the surface of theinitial glass film, said finely divided glass having a compositionsubstantially the same as that of the initial glass film and also havinga fusing point below the melting point of said metals, increasing thetemperature of said coating at a relatively slow rate until theelectrical resistance of the metal-glass system is reduced tosubstantially its minimum value, increasing the temperature of themetal-glass coating to at least the fusing point of the glass, but belowthe melting point of said metals, at a relatively rapid rate sufficientto avoid any substantial increase in the electrical resistance of themetal-glass system, whereby said metal-glass system penetrates into theinitial glass film, maintaining the metal-glass coating at the fusingtemperature for a period sutficient to completely fuse the glass withoutsubstantially increasing the electrical resistance of the metal-glasssystem, and cooling the resulting glassy coating to solidify :the glassand form a continuous impervious composite metal-glass film bonded tothe substrate.

13. A film-type electrical resistor comprising the combination of anelectrically insulating substrate which is resistant to hightemperatures, a glass coating deposited on the surface of said substrateand bonded thereto, and a metal-glass film integral with the upperportion of said glass coating and having a glass component ofsubstantially the same composition as said glass coating and with afusing point below the melting point of the metal component, said glasscomponent having been fused to form a continuous impervious metal-glassfilm integral with said glass coating, said metal component consistingessentially of silver, platinum, and gold.

14. A method of producing a film-type electrical resistor comprising thesteps of providing an electrically insulating substrate which isresistant to high temperatures, depositing a mixture of :finely dividedglass and thermally decomposable organic compounds of the metals goldand platinum as a thin coating on the surface of said substrate, saidglass having a fusing point below the melting point of said metal,increasing the temperature of said coating at a relatively slow rateuntil the electrical resistance of the metal-glass system is reduced tosubstantially its minimum value, increasing the temperature of saidcoating at least to the fusing point of said glass, but below themelting point of said metals, at a relatively rapid rate sufiicient toavoid any substantial increase in the electrical resistance of themetal-glass system, maintaining the coating at the fusing temperaturefor a period sufiicient to completely fuse the glass withoutsubstantially increasing the electrical resistance of the metal-glasssystem, and cooling the resulting glassy coating to solidify the glassand form a continuous impervious metal-glass film bonded to the surfaceof the substrate.

15. A film-type electrical resistor comprising the combination of anelectrically insulating substrate which is resistant to hightemperatures, and a metal-glass film decomponent with a fusing pointbelow the melting point of the metal component, said glas componenthaving been fused to form a continuous impervious metal-glass filmbonded to the surface of the substrate, and a metal component consistingessentially of gold and platinum 16. A film-type electrical resistorcomprising the combination of an electrically insulating substrate whichis resistant to high temperatures, a glass coating deposited on thesurface of said substrate and bonded thereto, and a metal-glass filmintegral with the upper portion of said glass coating and having a glasscomponent of substantially the same composition as said glass coatingand with a fusing point below the melting point of the metal component,said glass component having been fused to form a continuous imperviousmetal-glass film integral with said glass coating, said metal componentconsisting essentially of gold and platinum.

17. A method .of producing a film-type electrical resistor' comprisingthe steps of providing an electrically insulating substrate which isresistant to high temperatures, depositing a thin glass coating on thesurface of said substrate and fusing .the glass coating 50 as to form asmooth continuous glass film bonded to the substrate,

solidifying the glass coating, depositing a mixture of finely dividedglass and thermally decomposable organic compounds of the metals goldand platinum as a thin coating on the surface of the solidified glasscoating, said finely divided glass having substantially the samecomposition as the inital glass coating and also having a fusing pointbelow the melting point of said metals, increasing the temperature ofsaid metal-glass mixture to at least the fusing point of the glass, butbelow the melting point of said metals, so that the metal-glass mixturepenetrates into the initial glass coating forming a smooth surfacedcomposite metal-glass film, and cooling said composite film to solidifythe glass and form a continuous impervious metalglass film.

References Cited by the Examiner UNITED STATES PATENTS 2,357,473 4/1944Jira 33826 2 2,457,678 12/ 1948 Iira l172l7 2,461,878 2/1949 Christensenet al. 338-30 8 2,882,187 4/1959 Kwate 1172i 17 2,939,807 6/ 1960Needham 1172:1 7

RICHARD D. NEVIUS, Primary Examiner.

1. A METHOD OF PRODUCING A FILM-TYPE ELECTRICAL RESISTOR COMPRISING THESTEPS OF PROVIDING AN ELECTRICALLY INSULATING SUBSTRATE WHICH ISRESISTANT TO HIGH TEMPERATURES, DEPOSITING A MIXTURE OF FINELY DIVIDEDGLASS AND THERMALLY DECOMPOSABLE ORGANIC COMPOUNDS OF THE METALS SILVER,PLATINUM, AND GOLD AS A THIN COATING ON THE SURFACE OF SAID SUBSTRATE,SAID GLASS HAVING A FUSING POINT BELWO THE MELTING POINTS OF SAIDMETALS, INCREASING THE TEMPERATURE OF SAID COATING AT A RELATIVELY SLOWRATE UNTIL THE ELECTRICAL RESISTANCE OF THE METAL-GLASS SYSTEM ISREDUCED TO SUBSTANTIALLY ITS MINIMUM VALUE, INCREASING THE TEMPERATUREOF SAID COATING AT LEAST TO THE FUSING POINT OF SAID GLASS, BUT BELOWTHE MELTING POINT OF SAID METALS, AT A RELATIVELY RAPID RATE SUFFICIENTTO AVOID ANY SUBSTANTIALLY INCREASE IN THE ELECTRICAL RESISTANCE OF THEMETAL-GLASS SYSTEM, MAINTAINING THE COATING AT THE FUSING TEMPERATUREFOR A PERIOD SUFFICIENT TO COMPLETELY FUSE THE GLASS WITHOUTSUBSTANTIALLY INCREASING THE ELECTRICAL RESISTANCE OF THE METAL-GLASSSYSTEM, AND COOLING THE RESULTING GLASSY COATING TO SOLIDIFY THE GLASSAND FORM A CONTINUOUS IMPERVIOUS METAL-GLASS FILM BONDED TO THE SURFACEOF THE SUBSTRATE.